The present invention relates generally to the field of moisture absorption in hygroscopic materials, and more specifically to methods and apparatus for predicting moisture absorption in such materials.
Certain types of Plastic Encapsulated Microchip (PEM) packages that are exposed to humid air, prior to a reflow soldering process (which attaches these types of components to printed electronic circuit boards), are capable of absorbing significant quantities of water vapor. It is also known that some unprotected PEM packages are capable of absorbing so much moisture, due to their exposure to humidified air, that the rapid heating effect during subsequent soldering causes them to fracture, crack, or explode. xe2x80x9cPopcorningxe2x80x9d is a term that is often applied to describe this effect. Typically, this kind of damage may cause immediate, or somewhat delayed, fatal defects in the electrical operation of these PEMs. Additional information and much more detail regarding the popcorning effect, as it relates to PEMs, is available in many other previously published papers [e.g., Baluck et al., 1995; Fukuzawa et al., 1985; Gallo and Munamarty, 1995; Gannamani and Pecht, 1996; Holcomb et al., 1994; Hua and Leong, 1998; Hutchins, 1990; Ilyas and Poborets, 1993; Ito et al., 1991; Kitano et al., 1988; Mahajan and Pecht, 1999; McCluskey et al., 1997; Oizumi et al., 1987; Pecht and Govind, 1997; Pecht et al., 1997; Shan et al., 1992; Shook, 1992; Theriault et al., 2000; Totten, 1996A and 1996B; Tubbs and Gallo, 1996; Yalamananchiii et al., 19951].
The specific amount of water vapor that can be absorbed within different types of PEMs depends significantly upon the type and configuration of the plastic encapsulating material that is used to enclose the interior electronic component(s) as well as the exposure time and relative humidity (RH) of the moist air that these devices are exposed to. For these reasons, actual moisture absorption rates, associated with specific PEMs exposed to well defined RH and temperature conditions, must be measured experimentally. And, although there are at least a few experimental studies related to this problem within the extant literature, many of these studies have focused upon moisture absorption measurements involving temperatures and RH conditions that are unlikely, if not (for all practical purposes) impossible within most electronic packaging and assembly environments. However, one of the best previous studies, known to the inventors, involving moisture absorption rates of PEMs and also containing the most realistic operating conditions, was published in a Surface Mount Technology (SMT) Proceedings volume in 1992 [Chong et al., 1992]. In that paper, the lowest controlled operating temperature and relative humidity condition was at 30 C and 60% RH. In addition, theoretical equations allowing predictions at other temperatures and RH conditions were developed. The dried PEMs studied during that work were also exposed for long time periods (up to 1,000 hr) to high humidity test atmospheres. However, although moisture pick up behavior could be estimated at ambient temperature conditions, that behavior was not measured directly.
Related art may be found in the following, some of which are referred to herein:
ANSI/IPC-SM-786A, Procedures for Characterizing and Handling of Moisture/Reflow Sensitive ICs. IPC, Northbrook, Ill. Sep. (1994).
Baluck, M. J., Rose, G. L., and Virmnani, Fundamentals of Plastic Encapsulated Microcircuits (PEMs) for Space Applications. NASA TECDOCS (1995).
Chong, D., Dunn, C., Lewis, T., and LeBlanc, J., Moisture Sensitivity of Surface Mount Plastic Packages. Proceedings of the Technical Program: Surface Mount International Conference and Exposition, Edina, Minn. Vol. 1, pp. 421-426 (1992).
Fukuzawa, I., Ishiguro, S., and Nanbu, S., Moisture Resistance Degradation of Plastic LSIs by Reflow Soldering. Proceedings of International Reliability Physics Symposium, pp. 192-197 (1985).
Holcomb, K., Ryan, L., and Suro, E., Moisture in Liquid Epoxy Encapsulant and its Effect on SMDs. Dexter Corporation Technical Paper, Olean, N.Y. December (1994).
Intel Corp., Moisture Sensitivity/Desiccant Packaging/Handling of PSMCs, 1993.
IPC/JEDEC J-STD-020A, Moisture/Reflow Sensitivity Classification for Non-Hermetic Solid State Surface Mount Devices. IPC, Northbrook, Ill. (1999).
IPC/JEDEC J-STD-033, Standard for Handling, Packaging, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices. IPC, Northbrook, Ill. (1999).
Gallo, A. A. and Munamarty, R., Popcorning: A Failure Mechanism in Plastic Encapsulated Microcircuits. Dexter Corporation Technical Paper, Olean, N.Y. September (1995).
Gannamani, R. and Pecht, M., An Experimental Study of Popeorning in Plastic Encapsulated Microcircuits. IEEE Transactions on Components, Packaging, and Manufacturing Technology, Part A, Vol. 19, No. 2, pp. 194-201, June (1996).
Hua, F. and Leong, B., A Moisture Induced Failure in FCBGA Packages During Multiple Reflow. Proceedings of the Technical Program: Surface Mount International Conference and Exposition, San Jose, Calif. pp. 14-17 (1998).
Hutchins, C. L., Time and Temperature Requirements for Surface Mount Soldering. Proceedings of the Technical Program: NEPCON West, Anaheim, Calif. pp. 288-297 (1990).
Ilyas, Q. S. M. and Poborets, B., Evaluation of Moisture Sensitivity of Surface Mount Plastic Packages. Proceedings of the ASME Conference, New Orleans, La. pp. 145-156 (1993).
Ito, S., Nishioka, T., Oizumi, S., Ikemura, K., and Igarashi, K, Molding Compounds for Thin Surface Mount Packages and Large Chip Semiconductor Devices. Proceedings of the 39th International Reliability Physics Symposium, pp. 190-197 (1991).
Kitano, M., Nishimur, A., Kawai, S., and Nishi, K., Analysis of Package Cracking During Reflow Soldering Process. Proceedings of the 26th International Reliability Physics Symposium, pp. 90-95 (1988).
McCluskey, P., Munamarty, R., and Pecht, M., Popcorning in PBGA Packages During IR Reflow Soldering. Microelectronics International, Vol. 42, pp. 20-23, January (1997).
Pecht, M. and Govind, A., In-situ Measurements of Surface Mount IC Package Deformations During Reflow Soldering. IEEE Transactions on Components, Packaging, and Manufacturing Technology, Part C, Vol. 20, No. 3, July (1996).
Pecht, M., Ranade, Y., and Pecht, J., Effect of Delamination on Moisture Accelerated Failures in Plastic Encapsulated Microcircuits. Circuit World, Vol. 23, No. 4, pp. 11-15 (1997).
Shan, X., Agarwal, R., Pecht, M., and Evans, J., Effect of Humidity on Reliability of Overlaid High Density Interconnects. Proceedings IEEE Multichip Module age Conference, pp. 106-109, March (1992).
Shook, R. L., Moisture Sensitivity Characterization of Plastic Surface Mount Devices Using Scanning Acoustic Microscopy. Proceedings of International Reliability Physics Symposium, pp. 157-168 (1992).
Totten, C. R., Part I: Managing Moisture Sensitive Devices, Circuits Assembly, pp. 56-64, September (1996A).
Totten, C. R., Part II: Managing Moisture Sensitive Devices, Circuits Assembly, pp. 34-38, October (1996B).
Theriault, M., Carsac, C., and Blostein, P., Evaluating Nitrogen Storage as an Alternative to Baking Moisture/Reflow Sensitive Components. Proceedings of the Technical Program: NEPCON West, Anaheim, Calif. (2000).
Tubbs, T. R. and Gallo, A. A., Accelerated Popcorn Testing of High Solder Reflow Crack Resistant Molding Compounds. Dexter Corporation Technical Paper, Olean, N.Y. April (1996).
Yalamanchiii, P., Gannamani, R., Munamarty, R., McCluskey, P., and Christou, A., Optimum Processing Prevents Popcorning. Surface Mount Technology, pp. 39-42, May (1995).
It is thus desired to provide a method and apparatus for estimating moisture absorption by hygroscopic materials for obtaining a better understanding of the kinetics of the water absorption process that may take place at ambient temperatures, between air saturated (or nearly saturated) with moisture and certain initially dry PEM packages. It is an additional desire to provide a method and apparatus for estimating moisture absorption by hygroscopic materials that indicates the advantages of methods of storing PEM packages in order to minimize water absorption by the PEMs and thus inhibits the xe2x80x9cpopcorningxe2x80x9d effect during subsequent reflow soldering. It is a further desire to provide a method and apparatus for estimating moisture absorption by hygroscopic materials that provide a quantitative indication of specific mass increases due to moisture absorption in pre-dried PEM specimens. It is a still further desire to provide a method and apparatus for estimating moisture absorption by hygroscopic materials that enables the rapid development of complete xe2x80x9cfamiliesxe2x80x9d of absorption isotherms from a minimal amount of xe2x80x9crealxe2x80x9d experimental data.
It is thus one feature of the present invention to provide a method and apparatus for estimating moisture absorption by hygroscopic materials.
It is a further feature to of the present invention to provide a method and apparatus for estimating moisture absorption by hygroscopic materials that enable prediction of moisture absorption at various relative humidity conditions for a material from minimal data of moisture absorption at specific, controlled relative humidity and temperature conditions for that material.
Accordingly, a method and apparatus for estimating moisture absorption by hygroscopic materials is provided. The estimating method and apparatus includes: a substantially air tight container adapted to hold a test specimen therein. The atmospheric conditions within the container may be controlled by means such as gas injection and withdrawal. Further, the relative humidity of the atmosphere may be controlled by means such as, but not limited to, gas injection and/or humidifying material disposed within the container. It may be desired to include means for circulating the atmosphere within the container. The container may also include atmospheric sensing means to monitor atmospheric conditions within the container. Such conditions may include temperature and humidity.
One method of predicting the moisture absorption rate in a material includes the steps of: drying the material for a time sufficient to remove residual moisture; placing the material within a controlled atmosphere container; exposing the material to an environment of a known controlled relative humidity in an inert gaseous atmosphere and controlled temperature; collecting data of moisture absorption over time and using curve fitting technique to fit the data to a curve using the equation Y=aXb; where: a is a constant ranging from about 0.001 to about 1.0; b is a constant ranging from about 0.01 to about 10.0; Y is the mass increase in grams H20 per 100 grams of material; and X is humidification time in hours; wherein once constants a and b are found at said known controlled relative humidity and said controlled temperature for said material, assuming constant b is a constant value for the material, and constant a is a variable that is directly proportional to the relative humidity in an inert gaseous atmosphere; and modifying the variable a and holding the variable b constant to generate an expected moisture absorption mass gain versus time curve for a different specific relative humidity value.
Preferred are those methods wherein the known relative humidity ranges from 5 to 100%, and those methods wherein the material is a PEM or other electronic component.
Another aspect of the invention is an apparatus for creating a controlled moisture atmosphere, the apparatus comprising a sealed container adapted to contain a nearly saturated atmosphere of moist gas, the apparatus able to maintain said atmosphere by at least one wet fibrous material placed within a corresponding tray partially filled with water, the tray located inside of this container so the central region is available for holding a test specimen.
Preferred are apparatus wherein the moist gas may be circulated within the sealed container by a small internal electric fan. Also preferred are apparatus wherein the actual RH level and temperature within this controlled atmosphere chamber may be continuously monitored and recorded using RH and temperature probes inserted through hermetically sealed fittings welded into one end of the sealed container. In particularly preferred embodiments the RH and temperature probes may be combined in a single probe inserted through a single sealed fitting.
Other aspects of the invention will become apparent through reading the detailed description and claims which follow, which are illustrative only in nature.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description may be utilized singularly or in combination to achieve the invention as taught. Additional features and advantages to the invention will be described hereinafter which form the subject of the claims appended hereto.